Plastic pipes of the type described above, comprising one thin-walled inner pipe, are previously known. They are used, for example, as underground drain pipes, pressure pipes and cable ducts. They are more complicated to manufacture than conventional single-layer pipes, but since the consumption of material and thus also the weight of the pipe are lower than with single-layer pipes having corresponding properties, multilayer pipes are somewhat less expensive than conventional pipes. Nevertheless, their use is very limited particularly in northern latitudes. The main reason for this is that pipes of this kind have poor mechanical properties as compared with conventional single-layer pipes.
A conventional three-layer underground drain pipe has the following construction, which complies with the standards published in the field (the outside diameter of the pipe being 315 mm):
inner layer of hard PVC plastic, thickness about 1.4 mm, elasticity modulus about 2000 MPa, density about 1400 kg/M.sup.3, PA1 middle layer of foamed PVC plastic, thickness about 9.4 mm, elasticity modulus about 800 MPa, foam density about 800 kg/m.sup.3, PA1 outer layer of hard PVC plastic, thickness about 1.4 mm, elasticity modulus about 2000 MPa.
The ring stiffness of such a pipe is about 8.8 kN/m.sup.2, which is sufficient for underground laying. The ring stiffnesses of the different pipe layers are as follows: the inner pipe (1) about 0.0167 kN/m.sup.2, the middle layer (2) about 1.8 kN/m.sup.2, and the outer pipe (3) alone about 0.0136 kN/m.sup.2.
In the order of magnitude, the ring stiffnesses are 2, 1 and 3. This is the typical and predominant construction of foam pipes available on the market. Despite the foaming and thus the lowest elasticity modulus, the middle layer is the stiffest and the most load-bearing structure. The inner pipe is typically the second stiffest structure.
If the weights of the different layers of three-layer foam pipes available on the market are examined, another predominant dimensioning principle can be seen: the proportion of the weight of the solid layers to the total weight of the pipe is always less than 45%. In the example described above, the weight of the inner pipe was about 1.9 kg/m, of the middle layer about 7.4 kg/m, and of the outer pipe about 2 kg/m. The weight proportion of the inner pipe and the outer pipe taken together to the total weight of 11.2 kg was thus 34%.
U.S. Pat. No. 4,364,882 discloses a conventional PVC foam pipe. The PVC is foamed to a density of 500 kg/m.sup.3, which in fact is the lowest value obtainable by conventional techniques. The typical foaming degree of PVC foam is 57%, whereby the density is 800 kg/m.sup.3 ; if the density is lower than this, the strength properties of the PVC foam are weakened. The patent discloses a pipe having an outer diameter of 315 mm and the following structure: the thickness of the inner pipe 1.25 mm, the thickness of the middle layer 9 mm, and the thickness of the outer pipe 1.25 mm. The total thickness of the pipe is thus 11.5 mm, and the total weight is 7.63 kg/m; thus the pipe disclosed is 29% lighter in weight than a conventional pipe with a corresponding stiffness.
Further the following characteristics of the pipe above can be calculated: the weight of the inner pipe 1.61 kg/m and the ring stiffness 0.013 kN/m.sup.2 ; the weight of the middle layer 4.32 kg/m and the ring stiffness 1.41 kN/m.sup.2 ; the weight of the outer pipe 1.72 kg/m and the ring stiffness 0.011 kN/m.sup.2. It can be seen that, as compared with the outer and inner pipes, the ring stiffness of the foamed middle layer is more than hundredfold, and that the weight proportion of the outer and inner pipes taken together is 44%.
The example described above shows that the use of foamed plastic has the advantage that it saves considerable amounts of material (i.e. cost saving) and the pipe construction becomes lighter. Such a use of material of "poorer quality" in the middle layer is appropriate in this connection, since this layer is the least subjected to mechanical stresses, such as wear and stress strains, and to physical and chemical stresses, such as UV radiation and various impurities.
On the other hand, when the foaming degree of the middle layer is increased, or its density is reduced, the properties of the foamed material are significantly weakened. So far the highest foaming degree used has in practice reduced the density of the material to half of the density of unfoamed material. If a higher foaming degree were used, the strength of the foam would be considerably weakened, and it has been considered impossible to construct a pipe of good quality using such foam. Although in the prior art solutions the foaming, for instance, has been maintained within such a range that the foamed material still has relatively good mechanical strength properties, it has not always been possible to avoid damages extending as far as the inner pipe. Nor has the use of a foamed intermediate layer resulted in quite as significant cost savings as originally intended.